Apparatus and method for performing spinal surgery

ABSTRACT

An embodiment of an intervertebral prosthetic device for implantation in a spine includes a rigid fixation member and a compressible member. The fixation member is configured to be placed in a cavity of a first vertebral body and against bone of the first vertebral body. The compressible member is configured to be placed in a cavity in an intervertebral disc and to be secured to the first fixation member. Another embodiment includes two compressible members and one fixation member. In this embodiment, the first and second compressible members are sized to substantially replace the nucleus pulposus of first and a second intervertebral discs, respectively, on either side of a vertebral body. The fixation member is sized to fit within a cavity in the vertebral body between the first and second compressible members.

CROSS-REFERENCE TO RELATED APPLICATIONS

This invention claims priority to co-pending U.S. Provisional PatentApplication Nos. 60/492,966 (filed Aug. 7, 2003) and 60/512,186 (filedOct. 20, 2003).

BACKGROUND

This invention relates to the field of spinal surgery. Morespecifically, this invention relates to a novel implantable apparatusfor replacing the functionality of one or more failing intervertebraldiscs, without fusing the vertebral bodies above and below the disc(s).This invention also relates to devices for implanting and securing theintervertebral prosthetic device in cavities in a vertebral body and inone or more adjacent intervertebral discs. The invention further relatesto methods for performing spinal surgery.

The human spine is a flexible structure comprised of twenty-three mobilevertebrae. Intervertebral discs separate and cushion adjacent vertebrae.The top and bottom surfaces of intervertebral discs abut vertebral bodyendplates. The intervertebral discs act as shock absorbers and allowbending between the vertebrae.

An intervertebral disc comprises two major components: the nucleuspulposus and the annulus fibrosis. The nucleus pulposus is centrallylocated in the disc and occupies 25-40% of the disc's totalcross-sectional area. The nucleus pulposus usually contains 70-90% waterby weight and mechanically functions like an incompressible hydrostaticmaterial. The annulus fibrosis surrounds the nucleus pulposus andresists torsional and bending forces applied to the disc. Thus, theannulus fibrosis serves as the disc's main stabilizing structure. Ahealthy disc relies on the unique relationship of the nucleus andannulus to one another.

Individuals with damaged or degenerated discs often experiencesignificant pain. The pain results, in part, from instability in theintervertebral joint due to a loss of hydrostatic pressure in thenucleus pulposus, which leads to a loss of disc height and alteredloading of the annulus fibrosis.

A conventional treatment for degenerative disc disease is spinal fusion.In one such surgical procedure, a surgeon removes the damaged naturaldisc and then fuses the two adjacent vertebral bodies into one piece.The surgeon fuses the vertebral bodies by grafting bone between theadjacent vertebrae and sometimes using metal rods, cages, or screws tohold the graft in place until the graft heals.

Although spinal fusion may alleviate pain associated with degenerativedisc disease, it also results in loss of motion at the fused vertebraljoint. Lack of motion at the fused site puts additional stress on thediscs above and below the fusion. The additional stress may cause theadjacent discs to degenerate and produce pain, thereby recreating theoriginal problem.

To remedy the problems associated with spinal fusion, various prostheticdevices have been developed either to replace the entire disc (i.e., thenucleus pulposus and the annulus fibrosis) with a prosthetic joint or toreplace the nucleus pulposus of the damaged disc with a suitablebiomechanical equivalent. Unfortunately, the previous approaches havecertain limitations because conventional total disc replacement devicesand nucleus replacement devices disrupt tissues that will not heal.

In the case of total disc replacement surgery, existing prostheticdevices have met with limited success in reproducing the biomechanics ofa natural disc. Moreover, the anterior longitudinal ligament must besevered as part of the anterior approach by which the device isimplanted. Worse, the severing may span two vertebral bodies for a twolevel reconstruction, which can lead to lessened spinal function andstability. Further, total disc replacement devices require removal of asubstantial portion the disc and attachment to the adjacent vertebralbodies. The endplates of the vertebral bodies are nonuniform andtypically sclerotic, which prevents the close physical joining ofendplate and device surfaces required for bone ingrowth to provideadhesion and can lead to subsidence of the disc replacement device intothe bone of the vertebral bodies if the endplates are shaved for contourmatching. Moreover, the devices display limited motion. Specifically, asa result of the oversized implant relative to the narrow disc space,total disc replacement often results in a range of motion of only about3.8° to 4.6°. Such a limited range of motion is the equivalent of aspinal fusion, which is defined to be motion of less than about 5°.

For example, U.S. Pat. No. 4,759,769 to Hedman et al. discloses asynthetic disc having upper and lower plates hinged together. Althoughthe hinged disc allows forward bending between adjacent vertebrae, thehinged disc does not allow axial compression or lateral flexion. Nordoes it allow axial rotation of the vertebral column at the site of theimplant. Therefore, the Hedman et al. device lacks the biomechanics of anatural disc.

Likewise, the prosthetic disc device disclosed in U.S. Pat. No.4,309,777 to Patil does not replicate natural motion between adjacentdiscs. The Patil device includes two cups, one of which overlaps theother and is spaced from the other by springs. The cups move only in asingle axial dimension. Thus, the Patil device does not enable naturalflexion of the spine in any direction. In addition, the highlyconstrained motion of the Patil device can lead to high device/tissueinterface stresses and implant loosening.

In the case of nucleus replacement devices, historically these devicesrequired perforation or partial excision of the annulus to insert thedevice. Breaking the continuity of the annular ring precludes normalstress loading of the annulus, which may be necessary for later healing.Further, degeneration of the annulus, exacerbated by damage done duringimplantation, may also result in increased loads placed upon theimplant. Increased loads of this nature may lead to subsidence of thedevice into the vertebral body, device extrusion through the annulardefect, or expulsion from the nuclear space. Moreover, these problemsare exacerbated in the situation in which more than one disc is to bereplaced because any or all of the devices may develop these problems.These problems are particularly challenging in the lumbar spine, wherethe discs are most highly stressed due to high bearing requirements.

A remarkable intervertebral synthetic prosthetic device that greatlyreduces the problems associated with total disc replacement andconventional nucleus replacement devices is disclosed in U.S. Pat. No.5,827,328 (“the '328 patent”) to Buttermann. The Buttermann devicesexcise the nucleus pulposus while maintaining the biomechanicalfunctionality of the intact annulus fibrosis. Moreover, theintervertebral prosthetic device permits at least four degrees ofrelative motion between two vertebral bodies on either side of targetedintervertebral disc. These degrees of relative motion include sagittalbending, coronal bending, axial rotation, and axial compression.Moreover, the compressible member permits small increments oftranslational movement between the vertebral bodies (i.e., fifth andsixth degrees of relative motion, namely anterior-posterior translationand side-to-side, or lateral, translation).

FIG. 1 shows an embodiment of an intervertebral prosthetic device 10according to one embodiment of the '328 patent that is designed toreplace a damaged natural disc. This device 10 is implanted by makingholes in two adjacent vertebral bodies and boring through the nucleuspulposus of the intervertebral disc between the vertebral bodies. Theintervertebral prosthetic device 10 has a first fixation member 14, asecond fixation member 16, and a compressible member 18 that ispositioned between the first fixation member 14 and the second fixationmember 16. In addition to restoring the disc height, the compressiblemember 18 acts as a shock absorber to minimize impact loading and, thus,minimize device failure or vertebral fracture.

The first fixation member 14 is positioned in a first vertebral body 20.The second fixation member 16 is positioned within a second vertebralbody 22 adjacent the first vertebral body 20. Each fixation member 14,16 has an adjustable member 28, 30, respectively, and a support member32, 34, respectively. Controlling the height of the adjustable members28 and 30, along with selecting an appropriately sized support member,controls the “disc” height. The disc height is defined as the axialdistance between the vertebrae above and below the operative disc.

The adjustable member 28 of the first fixation member 14 has animaginary first longitudinal axis (shown by double-arrowed line A-A inFIG. 1) and adjustment elements 24 that allow adjustment of the heightof the adjustable member 28 substantially along its longitudinal axis.In the embodiment shown in FIG. 1, the second fixation member 16 isstructurally similar to the first fixation member 14, but inverted. Theadjustable member 30 of the second fixation member 16 has a secondlongitudinal axis (shown by double-arrowed line B-B) and adjustmentelements 26 that allow adjustment of the height of the adjustable member30 substantially along its longitudinal axis.

FIG. 4 shows one embodiment of the first fixation member 14. In theembodiment shown in FIG. 1, the second fixation member 16 isstructurally similar to the first fixation member 14, but inverted.Thus, the following discussion also applies to the second fixationmember 16.

The adjustable member 28 of the first fixation member 14 is adjustablein an axial direction by adjustment elements 24. The adjustment elements24 comprise telescopic struts extending between a first, outer plate 31and a second, inner plate 33. The outer plate 31 is farther from theoperative intervertebral disc and hence farther from the compressiblemember 18. In contrast, the inner plate 33 is closer to the operativeintervertebral disc area and hence closer to the compressible member 18.In the embodiment illustrated in FIG. 1, the outer plate 31 has abone-contacting surface 27, and the inner plate 33 has a surface 35 forpositioning against the support member 32.

The adjustment elements 24 adjust the distance between the firstbone-contacting plate 31 and the second plate 33, thus adjusting theheight of the adjustable member 28. A surgeon may adjust the telescopicstruts to increase the height of the adjustable member and thusmechanically pre-load the compressible member 18 to reproduce the axialcompression absorbed by a nucleus pulposus of a natural disc.Pre-loading the compressible member restores the intervertebral heightat the operative joint, restores the function of the annulus fibrosis.Pre-loading also assures close apposition of an ingrowth surface 27, 29of the device to bone 36, 38.

Each telescopic strut is provided with a lock screw 63 to adjust thelength of the strut 24 and hence control the height of the adjustablemember. The lock screw 63 may comprise, for example, a pin (not shown)that extends through both the telescoping portion 65 and the housingportion 67 of the strut 24. Each strut 24 is independently adjustable.FIG. 5 shows a top view of the second plate 33 of the adjustable member28. The adjustment elements 24 preferably are spaced equidistant fromeach other to enable specific height adjustment of various regions ofthe adjustable member.

The first and second fixation members 14 and 16 have porous portions,such as the bone-contacting surface 27, to permit bone ingrowth. In FIG.1, the bone-contacting surface 27 of the adjustable member 28 ispositioned against the subchondral bone of an endplate 36 of thesuperior vertebral body 20, and the bone-contacting surface 29 of theadjustable member 30 is positioned against the subchondral bone of anendplate 38 of the inferior vertebral body 22. Alternatively, abiocompatible fabric or suitable material may be wrapped around thefixation members to enable bone ingrowth. As another alternative, abiocompatible coating may be applied to the fixation members tofacilitate bone ingrowth. The prosthetic device of FIG. 1 does notrequire conventional mechanical attachments, such as pegs or screws, tohold the prosthesis permanently in place. The intravertebral (i.e.,within a vertebral body) positioning of the fixation members 14, 16maintains the prosthetic device 10 in stable relationship at theoperative intervertebral joint. The prosthetic device, however, mayinclude mechanical or other attachments to supplement the porousportions of the fixation members and to temporarily fix the prostheticdevice in place until bone ingrowth has occurred.

To further promote bone ingrowth, the adjustment elements 24 may includefins 66 extending outward from an exterior surface of the element 24, asshown in FIG. 4. The fins 66 increase the surface area of the fixationmember 14 to which bone may attach. Preferably, these fins 66 arelocated on the adjustment elements that are positioned on the anteriorside of the adjustable member 28. The prosthetic device also may includeprotuberances (not shown) on the bone-contacting surface of theadjustable members to increase the surface area of the porous portion ofthe fixation members and, thus, encourage bone ingrowth.

FIG. 6 shows a cross-section of support member 32. The support member 32has a first surface 72 that operably faces away from the compressiblemember 18 and a second surface 74 that operably faces towards thecompressible member 18. The first and second surfaces 72 and 74 areoblique so that a circumferential surface 77 around the support member32 varies in width, as shown in FIG. 4. Thus, the support member 32 iswedge-shaped. In other words, the support member 32 preferably tapersfrom a maximum thickness at one side 73 to a minimum thickness at anopposite side 75. Generally, the support member 32 is thicker on theside of the fixation member 14 placed anteriorly in a patient's spine toaccount for the spine's natural curvature.

The support members are constructed with various thicknesses and withvarious angled surfaces, depending upon the vertebral level of theoperative intervertebral joint. An angle θ shown in FIG. 6 rangesbetween 3°-10°. The support members are shaped to maintain sagittalalignment. Maintaining sagittal alignment avoids nonuniform loading ofthe compressible member and avoids early fatigue failure of that member.

The compressible member 18, which is shown in FIG. 2, can comprise atleast one spring and, in the illustrated embodiment, comprises aplurality of springs 40. The compressible member 18, which is implantedin the region of an excavated nucleus pulposus of the operativeintervertebral disc, is dimensioned so that the annulus fibrosis of thenatural disc is at least substantially (if not completely) maintained.As a result, the intervertebral prosthetic device restores themechanical properties of the disc without disrupting the annulusfibrosis. Retention of the annulus fibrosis maintains stability of theintervertebral joint at the implant site. In addition, the annulusfibrosis serves as a boundary for the compressible member and,therefore, minimizes the potential for accidental dislodgment of theprosthetic device.

The compressible member 18 has a top plate 42, a bottom plate 44, and aplurality of coil springs 40 extending between the top plate 42 and thebottom plate 44. The top plate 42 has a first surface 46, which isconnectable to the first fixation member 14, and a second surface 48.The bottom plate 44 also has a first surface 50, which is connectable tothe second fixation member 16, and a second surface 52. The springs 40extend between the second surfaces 48 and 52 of the top plate 42 andbottom plate 44, respectively.

When pre-loaded, the compressible member 18 can have an axial height ofapproximately 1.5 cm, greatest at the L45 vertebral level and slightlyless at the upper lumbar vertebrae. The coil springs 40 can havenon-linear stiffness so that they become stiffer at higher appliedloads; the nonlinear stiffness simulates physiological intervertebralstiffness. Moreover, any spring arrangement may be used that achievessufficient axial compressive stiffness to replicate the biomechanics ofthe natural disc.

The compressible member includes an imaginary longitudinal axis (shownby the dashed line C-C) and an outer periphery in a plane transverse toits longitudinal axis. A largest dimension of the compressible member'souter periphery is less than or substantially equal to the diameter of anucleus pulposus of the natural intervertebral disc. Put another way,the annulus fibrosis of the natural disc, which is substantially (if notcompletely) preserved during the implantation procedure, circumscribesthe compressible member 18. For example, where the compressible membercomprises a plurality of springs, the outer periphery of thecompressible member circumscribes the springs, and the largest dimensionof that outer periphery may extend to, but does not extend beyond, thenucleus pulposus. In other embodiments, where the compressible memberincludes a top plate and a bottom plate, and where those plates fitwithin the annulus fibrosis and extend beyond the outermost portions ofthe springs, the outer periphery of the compressible member equals thelarger of the two plate peripheries. In quantitative terms, the outerperiphery of the compressible member preferably ranges between 2.0 cm to3.0 cm, which approximates the diameter of the nucleus pulposus of anatural intervertebral disc.

FIGS. 3A-3C show three embodiments of a coil spring designed to possessnon-linear stiffness. In the embodiment of FIG. 3A, the coil spring 54has a variable, or non-uniform, cross-sectional diameter 56. FIG. 3Bshows another embodiment in which a coil spring 58 has a variable pitch60, where the pitch is defined as the distance between successive coilsof the spring 58. FIG. 3C shows a third embodiment of a coil spring 62in which at least two of the spring coils have different radii 64measured from an imaginary axis D-D extending along the central axis ofthe spring 62.

A method of intervertebral disc replacement now will be described inconnection with FIGS. 8-14. FIG. 8 shows a pathological intervertebraldisc 90 located between a superior vertebral body 92 and an inferiorvertebral body 94. Prior to implantation, a surgeon performs a partialvertebrectomy to excise bone matter from the superior vertebral body 92for receipt of a fixation member. This procedure can be performed usinga cutting guide and reamer. Bone harvested from the vertebral body 92 bythe reamer can be used after implantation of the prosthetic device topromote bone ingrowth into the prosthetic device, as later described.The partial vertebrectomy creates a cavity bounded by subchondral boneof a distant endplate 96 and subchondral bone of a near endplate 98 ofthe superior vertebral body 92. FIG. 9 shows a cross-sectional view ofthe superior vertebral body 92 after the partial vertebrectomy, as takenalong line 9-9 in FIG. 8.

The surgeon next excises the nucleus pulposus of the damaged disc tocreate a cavity 100, as shown in FIG. 10, for receipt of thecompressible member. The annulus fibrosis 102, seen in FIG. 11, ismaintained.

Upon completion of the partial vertebrectomies, the surgeon implants afixation member 104 into the inferior vertebral body 94, as shown inFIG. 11. The surgeon can select a support member with an appropriatethickness to accommodate the angulation at the operative intervertebrallevels. The surgeon then inserts a compressible member 106 (via thecavity formed in the superior vertebral body 92) into the cavityformerly containing the nucleus pulposus of the damaged disc andconnects it to the inferior fixation member 104, as shown in FIG. 12.The compressible member 106 and the fixation member 104 may be connectedby conventional attachment members, such as screws, or by biocompatiblecement or a suitable adhesive composition. Finally, the surgeon implantsanother fixation member, similar to the one implanted in the inferiorvertebral body 94, yet inverted, in the superior vertebral body 92.Connection of that fixation member to the compressible member 106 formsan intervertebral prosthetic device like the one shown in FIG. 1.

Once the fixation members are in place, the surgeon expands eachadjustable member. The surgeon applies distraction until the adjustablemember is seated against the subchondral bone and distant endplate 96 ofthe vertebral body and until the desired compression has been applied tothe compressible member. The adjustment elements of the adjustablemember are then secured, e.g., FIG. 13 shows rotation of the lock screws112 of individual telescopic struts 108 to secure the struts at anappropriate height.

The surgeon next packs cancellous bone grafts 118, typically obtainedduring creation of the cavity in the vertebral body, around the strutsof each adjustable member, as shown in FIG. 14. The growth of bonearound the fixation member and into its porous surfaces secures theintervertebral prosthetic device in place, absent mechanical attachmentstypically used in conventional disc prostheses. The surgeon thenreplaces the cortical bone from the partial vertebrectomy procedure and,if needed, secures it with a bone screw, suture or bone cement. Incertain clinical situations, as when there is poor bone healing orinsufficient bone, the surgeon may elect to use bone cement to attachthe fixation members to the vertebrae.

Although the embodiment shown in FIGS. 1-6 is effective, in someinstances it may be unnecessarily invasive as a result of itsimplantation via two vertebral body holes. FIG. 7 shows a second, lessinvasive embodiment described in the '328 patent, in which a prostheticdevice is implanted via one vertebral body hole.

FIG. 7 shows an intervertebral prosthetic device 76 according to thissecond embodiment that comprises a first fixation member 78, a secondfixation member 80, and a compressible member 82. The compressiblemember 82 is positioned between the first and second fixation members78, 80. The second fixation member 80 comprises a wedge-shaped supportmember with an upper surface 84 that attaches to the compressible member82 and a lower surface 86 that rests upon subchondral bone of a nearendplate 88 of an inferior vertebral body. In this embodiment,adjustment of the first fixation member 78 pre-loads the compressiblemember 82 to an appropriate extent. Also, in this embodiment, a lowersurface 86 of the support member 80 may be composed of a porous materialand may have a slightly convex shape to match the natural contour of thenear endplate of the inferior vertebral body.

The implantation of the FIG. 7 embodiment is similar to the implantationof the FIG. 1 embodiment. Specifically, similar to the embodiment shownin FIG. 10, a cavity may be formed in the superior vertebral body 92 andthen extended through the nucleus pulposus of the intervertebral disctherebelow. At this time, the compressible member with the lowerfixation member 80 affixed thereto may be inserted through the cavity inthe vertebral body and then pushed downward into the cavity 100 in theintervertebral disc. Subsequently, the upper fixation member 78 is: (a)positioned in the cavity formed in the superior vertebral body 92; (b)connected to the compressible member; and (c) adjusted in the mannerprevious discussed with respect to the FIG. 1 embodiment. Of course, thecavity in the superior vertebral body 92 is then closed also in themanner previously described.

As evident from the embodiments of FIGS. 1 and 7, the intervertebralprosthetic device embodiments have a modular design so that theprosthesis may be sized to the patient's anatomy and designed for thepatient's condition. The modular design also enables replacement ofindividual components of the prosthesis (i.e., a fixation member or acompressible member), rather than replacement of the entire prosthesisshould one component fail. As a result, the compressible member can beattached to the fixation members by mechanical attachments, such asscrews, rather than bone cement so that a surgeon may easily replacedamaged or worn components.

Unfortunately, the embodiment shown in FIG. 1 precludes use whenreconstructing multiple adjacent discs. Additionally, although the lessinvasive embodiment shown in FIG. 7 may be implanted via only onevertebral body hole, it may be less effective than the embodiment shownin FIG. 1 when used in patients with low bone density. Specifically, theFIG. 7 may be less effective as a result of inability to adequately fixthe lower fixation member 80 to the vertebral body below thecompressible member. Further, this inability to adequate fix the lowerfixation member 80 may, in turn, lead to subsidence of the device intothe vertebral body adjacent the lower fixation member 80.

SUMMARY

An embodiment of the invention addresses a prosthetic device thatincludes: a first compressible member sized to substantially replace thenucleus pulposus of a first intervertebral disc; a second compressiblemember sized to substantially replace the nucleus pulposus of a secondintervertebral disc that is separated from the first intervertebral discby a vertebral body; and a fixation member sized to fit within a cavityin the vertebral body between the first and second compressible members.

Another embodiment of the invention addresses a prosthetic device thatincludes: a fixation member sized to fit within a cavity in a firstvertebral body; and a compressible member sized to substantially replacea nucleus pulposus of an intervertebral disc adjacent the vertebralbody. A first side of the compressible member is configured to engagethe fixation member and a second side of the compressible member isconfigured to engage a second vertebral body. The second side of thecompressible member is configured to fit within a seat formed in thecortical bone of the endplate of the second vertebral body.

Another embodiment of the invention addresses an intervertebralprosthetic device for implantation in a spine that includes: (a) a rigidfixation member having a fixed length, the rigid fixation member beingconfigured to be placed in a cavity of a vertebral body and against boneof the vertebral body; and (b) a first compressible member configured tobe placed in a cavity in a first intervertebral disc adjacent thevertebral body and to be secured to the rigid fixation member. Thecompressible member is constructed to remain compressible afterimplantation and has at least one compressible element that remainscompressible after implantation. The rigid fixation member is sized tocompress the compressible member a predetermined amount when the rigidfixation member and the first compressible member are placed in thecavity in the first vertebral body and in the cavity in the firstintervertebral disc, respectively.

Another embodiment of the invention addresses an intervertebralprosthetic device for implantation in a spine. This device includes: (a)a fixation member configured to be placed in a cavity of a vertebralbody, the fixation member including: (i) an outer member configured tobe placed against bone of the vertebral body; (ii) an inner memberopposite the outer member; and (iii) at least one adjustment elementthat extends between the outer and inner members and that is configuredto adjust a length dimension of the fixation member along itslongitudinal axis; (b) a compressible member configured to be placed ina cavity in an intervertebral disc adjacent the vertebral body andconfigured to be secured to the inner member of the fixation member; and(c) a spacer sized to fit between the outer and inner members of thefixation member to maintain the fixation member at a desired lengthdimension.

Another embodiment of the invention addresses an intervertebralprosthetic device for implantation in a spine. This device includes: (a)a fixation member configured to be placed in a cavity of a vertebralbody, the fixation member including: (i) an outer member configured tobe placed against bone of the vertebral body; (ii) an inner memberopposite the outer member; and (iii) a longitudinal axis extendingbetween the outer and inner members; and (b) a compressible memberconfigured to be placed in a cavity in an intervertebral disc and to besecured to the inner member of the fixation member. The compressiblemember is constructed to remain compressible after implantation. Theouter member includes a tab extending outward along an axis differentfrom the longitudinal axis.

Another embodiment of the invention addresses an intervertebralprosthetic device for implantation in a spine. This device includes: (a)a fixation member configured to be placed in a cavity of a vertebralbody, the fixation member including: (i) an outer member configured tobe placed against bone of the vertebral body; (ii) an inner memberopposite the outer member; and (iii) a longitudinal axis extendingbetween the outer and inner members; (b) a compressible memberconfigured to be placed in a cavity in an intervertebral disc adjacentthe vertebral body and to configured be secured to the fixation member;and (c) at least one anchor element configured to immobilize and/orstabilize the compressible member and/or the fixation member.

Another embodiment of the invention addresses a prosthetic device thatincludes: (a) a fixation member sized to fit within a cavity in a firstvertebral body; and (b) a compressible member that includes: (i) acup-shaped base member; (ii) an upper member; and (iii) one or morecompressible elements provided between the base member and the uppermember. The compressible member is sized to substantially replace anucleus pulposus of an intervertebral disc adjacent the vertebral body.

Another embodiment of the invention addresses a prosthetic device thatincludes: (a) a fixation member sized to fit within a cavity in a firstvertebral body; and (b) a compressible member that includes: (i) a basemember; (ii) an upper member that includes a spike; and (iii) one ormore compressible elements provided between the base member and theupper member. The one or more compressible elements are sized tosubstantially replace a nucleus pulposus of an intervertebral discadjacent the vertebral body.

Another embodiment of the invention addresses a prosthetic device thatincludes: (a) a fixation member sized to fit within a cavity in a firstvertebral body; and (b) a compressible member that includes: (i) a basemember; (ii) an upper member comprising a ball-and-socket joint; and(iii) one or more compressible elements provided between the base memberand the upper member. The compressible member is sized to substantiallyreplace a nucleus pulposus of an intervertebral disc adjacent thevertebral body.

Another embodiment of the invention addresses a prosthetic device thatincludes: (a) a fixation member sized to fit within a cavity in a firstvertebral body; and (b) a compressible member that includes: (i) a basemember; (ii) an upper member; and (iii) one or more compressibleelements provided between the base member and the upper member. Thecompressible member is sized to substantially replace a nucleus pulposusof an intervertebral disc adjacent the vertebral body. The base memberis adjustable in a radial direction.

Another embodiment of the invention addresses a method of spinalprosthetic implantation. This method includes: (a) creating a cavity ina first vertebral body; (b) cutting a first hole through either a loweror an upper endplate of the vertebral body and through the nucleuspulposus of a first intervertebral disc adjacent thereto, therebycreating a first opening in the first intervertebral disc; (c) cutting asecond hole through the other of the lower and upper endplate of thevertebral body and through the nucleus pulposus of a secondintervertebral disc adjacent thereto, thereby creating a second openingin the second intervertebral disc; (d) implanting a first compressiblemember into one of the first and second openings; (e) implanting asecond compressible member into the other of the first or secondopenings; and (f) implanting a fixation member into the cavity in thefirst vertebral body.

Another embodiment of the invention addresses a method of spinalprosthetic implantation. This method includes: (a) creating a cavity ina first vertebral body; (b) cutting through an endplate of the vertebralbody and through the nucleus pulposus of an adjacent intervertebraldisc, thereby creating an opening in the intervertebral disc; (c)cutting into the cortical bone of a second vertebral body on the otherside of the intervertebral disc to create a seat; (d) implanting acompressible member into the opening in the intervertebral disc suchthat a distal end of the compressible member sits within the seat in thesecond vertebral body; and (e) implanting a fixation member in thecavity in the first vertebral body.

Another embodiment of the invention addresses a method of spinalprosthetic implantation. This method includes: (a) creating a cavity ina vertebral body; (b) cutting a hole through either a lower or an upperendplate of the vertebral body and through the nucleus pulposus of anintervertebral disc adjacent thereto, thereby creating an opening in theintervertebral disc; (c) implanting a compressible member into theopening in the intervertebral disc; and (d) implanting a fixation memberinto the cavity in the first vertebral body. The compressible membercomprises a base member that is wider than the hole cut in the vertebralbody through which the first compressible member is implanted. The stepof implanting the compressible member into the opening includes: (i)maneuvering the base member of the compressible member so that it passesthrough the hole and into the opening; and (ii) rotating the base memberso that it substantially covers the hole.

Another embodiment of the invention addresses a method of spinalprosthetic implantation. This method includes: (a) creating a cavity ina vertebral body; (b) cutting a hole through either a lower or an upperendplate of the vertebral body and through the nucleus pulposus of anintervertebral disc adjacent thereto, thereby creating an opening in theintervertebral disc; (c) implanting a compressible member into theopening in the intervertebral disc; and (d) implanting a fixation memberinto the cavity in the first vertebral body. The compressible membercomprises a base member that is radially adjustable to be wider than thehole cut in the vertebral body through which the compressible member isimplanted. The step of implanting the compressible member into theopening includes: (i) maneuvering the base member of the compressiblemember so that it passes through the hole and into the opening; and (ii)radially adjusting the base member so that it substantially covers thehole.

Another embodiment of the invention addresses a drill guide for use inspinal surgery. The drill guide includes a body having a first leg and asecond leg. The first leg is dimensioned to be fixed relative to anintervertebral prosthetic member mounted in a cavity of a firstvertebral body. The second leg is dimensioned to extend from the firstleg, adjacent the first vertebral body, to a free end, adjacent at leastone of an intervertebral disc and a second vertebral body. The secondleg includes at least one drilling channel extending through the secondleg and the free end of the second leg comprises a drill positioningblock through which the at least one drilling channel extends.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate a presently preferred embodimentof the invention, and, together with the general description given aboveand the detailed description of the preferred embodiment given below,serve to explain the principles of the invention.

FIG. 1 is a schematic, cut-away side view of a prior art intervertebralprosthetic device implanted in a spine.

FIG. 2 is a top perspective view of a compressible member of theintervertebral prosthetic device of FIG. 1.

FIGS. 3A-3C are top perspective views of different embodiments of aspring of the compressible member shown in FIG. 1.

FIG. 4 is a top perspective, partially exploded view of a fixationmember of the intervertebral prosthetic device of FIG. 1 and shows anadjustable member and a support member.

FIG. 5 is a top view of a plate of the adjustable member shown in FIG.1.

FIG. 6 is a side view, in cross-section, of the support member shown inFIG. 1.

FIG. 7 is a schematic, cut-away side view of another prior artintervertebral prosthetic device implanted in a spine.

FIG. 8 is a schematic, cut-away side view showing subchondral bones of asuperior vertebral body after a partial vertebrectomy.

FIG. 9 is a sectional view of a vertebra after creating a cavity withinthe vertebral body, as taken along line 9-9 of FIG. 8.

FIG. 10 is a schematic, cut-away side view of a vertebral joint areaafter creating a cavity within the vertebral body and excision of anucleus pulposus of a natural disc.

FIG. 11 is a schematic, cut-away side view of a vertebral joint andshows a fixation member, including an adjustable member and a supportmember, implanted in an inferior vertebral body.

FIG. 12 is a schematic, cut-away side view of a vertebral joint andshows a compressible member implanted in an intervertebral joint.

FIG. 13 is a schematic, cut-away side view of a vertebral joint andshows a technique for adjusting the height of an adjustable memberimplanted in a superior vertebral body.

FIG. 14 is a schematic, cut-away side view of a vertebral joint andshows a technique for bone grafting an adjustable member in a superiorvertebral body.

FIG. 15A is a top perspective view of an intervertebral prostheticdevice and a drill guide used to drill holes in the intervertebral bonein accordance with the invention; FIGS. 15B-15D are a first sideelevation view, a second side elevation view, and a top plan view,respectively, of the intervertebral prosthetic device and the drillguide of FIG. 15A.

FIGS. 16A-16D are a top perspective view, a first side elevation view, asecond side elevation view, and a top plan view, respectively, of thedrill guide of FIG. 15A.

FIG. 17A is a top perspective view of the intervertebral prostheticdevice and the drill guide of FIG. 15A, showing placement of anchorelements through the drill guide and a compressible member of theintervertebral prosthetic device; FIGS. 17B-17D are a first sideelevation view, a second side elevation view, and a top plan view,respectively, of the intervertebral prosthetic device, including theanchor elements, and the drill guide of FIG. 17A.

FIG. 18A is a side elevation view of an intervertebral prostheticdevice, including anchor elements, and a drill guide in accordance withanother embodiment of the invention; FIGS. 18B-18D are a top perspectiveview, a side elevation view, and a top plan view, respectively, of theintervertebral prosthetic device, including the anchor elements, and thedrill guide of FIG. 18A.

FIGS. 19A-19E are a top perspective view, a bottom plan view, a bottomperspective view, a first side elevation view, and a second sideelevation view, respectively, of a plate of an embodiment of a fixationmember in accordance with the invention.

FIG. 20 is a sectional view of a vertebra, including a plate, as shownin FIGS. 19A-19E, implanted in the vertebra.

FIG. 21A is a top perspective view of an intervertebral prostheticdevice, including anchor elements and spacers, in accordance withanother embodiment of the invention; FIGS. 21B-21D are a first sideelevation view, a second side elevation view, and a top plan view,respectively, of the intervertebral prosthetic device, including theanchor elements and the spacers, of FIG. 21A.

FIG. 22A is a top perspective view of an intervertebral prostheticdevice, including spacers, in accordance with another embodiment of theinvention; FIGS. 22B-22D are a first side elevation view, a second sideelevation view, and a top plan view, respectively, of the intervertebralprosthetic device, including the spacers, of FIG. 21A.

FIGS. 23A-23D are a top perspective view, a first side elevation view, asecond side elevation view, and a top plan view, respectively, of aspacer.

FIG. 24 is a front perspective view of another embodiment of animplantable prosthetic device; the device be shown in a break-away viewof a spine.

FIG. 25 is a top perspective view of the implantable device of FIG. 24.

FIG. 26 is a side perspective view of the implantable device of FIG. 24.

FIG. 27 is another side perspective view of the implantable device ofFIG. 24.

FIG. 28 is a perspective view of the implantable device of FIG. 24having anchor elements therethrough.

FIGS. 29A and 29B are respective cross-sectional and perspective viewsof an alternate embodiment compressible member; FIGS. 29C and 29D arerespective cross-sectional and perspective views of another alternateembodiment compressible member; and FIG. 29E is a cross-sectional viewof either of the embodiments shown in FIGS. 29A-29D implanted in anintervertebral disc.

FIGS. 30A and 30B are cross-sectional views of another alternateembodiment compressible member in which FIG. 30A shows the compressiblemember partial implanted and FIG. 30B shows the compressible membercompletely implanted; FIGS. 30C and 30D show an alternate embodimentplate that may be used in the embodiment shown in FIGS. 30A and 30B;

FIG. 31 is a cross-sectional view of two compressible members implantedin intervertebral discs, one of which is another alternate embodimentcompressible member.

FIG. 32 is a cross-sectional view of alternate embodiment compressiblemember implanted in an intervertebral disc.

FIGS. 33A and 33B are a top perspective view and a side elevation view(in cross section), respectively, of an endplate and nucleus cutter;FIG. 33C is a side elevation view, in cross section, of an alternativeembodiment of the endplate and nucleus cutter.

FIG. 34A is a side elevation view of a compressor that includes a pairof endplate and nucleus cutters and FIG. 34B is a side elevation view ofa distractor that includes an endplate and nucleus cutter.

FIG. 35 is a schematic view of a compressor with endplate and nucleuscutters inserted into the cavities of adjacent vertebral bodies.

FIG. 36A is a schematic view of an alternative embodiment of theendplate and nucleus cutter in accordance with the invention; FIG. 36Bis a bottom plan view of a main body of the endplate and nucleus cutterof FIG. 36A; FIGS. 36C and 36D are a top perspective view and a bottomplan view, respectively, of the cutting surface of the endplate andnucleus cutter of FIG. 36A; and FIG. 36E is a perspective view ofanother alternative embodiment of the endplate and nucleus cutter.

DETAILED DESCRIPTION

Although the less invasive embodiment shown in FIG. 7 may not be aseffective as the embodiment shown in FIG. 1 and/or may be subject tosubsidence, the ability to implant a disc replacement prosthetic devicevia a hole formed in only one adjacent vertebral body is minimallyinvasive and is, therefore, advantageous. As a result, the questionbecomes: how can one replace one or more discs via one vertebral bodyhole while: (a) greatly reducing the likelihood of subsidence, (b)making the device adaptable to particular patients and/or to theparticular disc being replaced; (c) ensuring that the device remains inproper position; (d) providing a straightforward method of implantation;(e) making it cost effective for the patient.

The answer to one or more of the parts to this question lies in theprosthetic device embodiments disclosed herein. These prosthetic deviceembodiments are not only readily implantable via a hole in a singlevertebral body, they are so implantable while: (a) reducing thelikelihood subsidence by means of anchors and tabs which serve to fixthe device to the cortical bone of the vertebral body; (b) beingadaptable by means of various compressible member embodiments; (c)ensuring the device remains in position by being fixedly engaged withthe vertebrae and by being encapsulated by the annulus fibrosis; (d)being implantable in a time frame which is no longer than currentimplantation surgery; and (e) being cost effective by means of beingmodular and adjustable. The various embodiments of this novel prostheticdevice will now be described with reference to the drawings, whereinlike numerals indicate like parts.

A prosthetic device 200 that minimizes the likelihood that the devicemay subside in patients with osteopenic bone is shown in FIGS. 15A-15Dand 17A-17D. In certain patients having thin bones, the bone (labeled36, 38 in FIG. 1, and labeled 88 in FIG. 7) in areas adjacent to theimplanted prosthetic device may be subject to being crushed and/orcollapse under heavy loads. This condition also may occur in otherpatients after implantation of the prosthetic device, but before thebone graft has time to heal. To minimize subsidence, one or more anchorelements, such as pins, rods, or screws, can be threaded either througha fixation member or through the compressible member to fix theprosthetic device in the hard outer cortical bone of the vertebra inwhich the cavity was created for device implantation.

The following description of FIGS. 15A-15D and 17A-17D relates to aprosthetic device embodiment having a compressible member and only onefixation member. However, it should be understood that the featuresdescribed below can be applied to a prosthetic device having a fixationmember on either side of the compressible member (as shown in FIGS.21A-21D) or to a prosthetic device having two compressible members oneither side of a fixation member (as shown in FIGS. 25-28).

The one compressible member/one fixation member embodiment of FIGS.15A-15D, 17A-17D is similar to the embodiment of FIG. 7. However,whereas the FIG. 7 embodiment provides for a separate second fixationmember, the embodiment of FIGS. 15A-15D, 17A-17D incorporates at leastsome of the functionality of the second fixation member into thecompressible member, amongst other improvements as later described indetail.

FIGS. 15A-15D and 17A-17D show a prosthetic device 200 having a fixationmember 214 for fixation within a cavity of a first vertebral body, and acompressible member 218 for implantation in the region of an excavatednucleus pulposus of an operative intervertebral disc. In thisembodiment, the fixation member 214 comprises an adjustable memberhaving an outer plate 250 and an inner plate 252. The compressiblemember 218 includes a first plate 242, a second plate 244, and one ormore compressible elements. In the embodiment shown, the compressibleelements are a plurality of coil springs 240, positioned between theplates 242, 244. Adjacent plates of the adjustable fixation member 214and the compressible member 218 can be secured directly together. Thatis, an inner plate 252 of the fixation member 214 can include an angledprotrusion 253 that mates with an angled recess 243 in the first plate242 of the compressible member 218 in the manner of a dovetail joint'stenon and mortise. The angled portions 253, 243 secure the fixationmember 214 and the compressible member 218 together in a keyed fit,without need for other fasteners or fastening materials. It will beunderstood that, in an alternative embodiment, the angled protrusion canbe formed on the first plate of the compressible member, and the angledrecess can be formed in the inner plate of the fixation member.

As previously mentioned, the prosthetic device 200 is designed tominimize subsidence of the device 200 into bone adjacent the device 200by employing anchor elements to secure the device 200 into hard outercortical bone. In the illustrated embodiment, plate 244 of thecompressible member 218 includes holes 246 for receipt of anchorelements 310. A drill guide can be used to create holes through thecortical bone toward the holes 246 in the device 200. FIGS. 16A-16D showan embodiment of the drill guide 300. The drill guide 300 includes anL-shaped body 302 having a curved face 304 at one end and a drillpositioning block 306 at the other end. The drill positioning block 306can have one or more drilling channels 308. The drilling channels 308are configured to guide a drill bit through the bone toward the holes246 in the plate 244 of the compressible member 218.

As best seen in FIG. 15A, the drill guide 300 can temporarily engageprosthetic device 200 to guide the drilling of holes through bone towardholes 246 in prosthetic device 200. The guide 300 subsequentlyfacilitates placement of the anchor elements 310 through the holes 246,as best seen in FIG. 17A. To properly position the drill guide 300relative to prosthetic device 200, an upper portion 301 of the L-shapedbody 302 is pushed between two adjustment elements 224 of the fixationmember 214 such that the curved face 304 aligns with and engages a thirdadjustment element 224 of fixation member 214, as shown in FIG. 15A. Asthe upper portion 301 of the L-shaped body 302 has a width 303 which issubstantially equal to the distance between the two adjustment elements224 through which it is pushed, when the curved face 304 meets the thirdadjustment elements 224, the three adjustment elements 224 substantiallyimmobilize the drill guide 300 with respect to the device 200.

Other embodiments of the drill guide need not include curved face 304and can be configured to mount to other portions of the prostheticdevice. In addition, the L-shaped body of the drill guide 300 can beconfigured to be adjustable along each of the two legs that form the“L”. For example, each leg of the “L” can comprise telescoping elementsto lengthen or shorten the leg, depending on the size of the prostheticdevice and the position of holes 246 of the compressible member 218.Further, as an alternative to mounting the drill guide 300 to theprosthetic device 200, the guide 300 can be mounted to the outervertebral body set to receive fixation member 214. In addition, where itis inconvenient or undesirable to use a drill guide 300, anothertechnique, such as fluoroscopic imaging, may be used to determine drillplacement. However, such a protocol may be less accurate.

Once holes have been made in the bone, anchor elements 310 can beinserted through drilling channels 308, through the newly-drilled holesin the bone, and through the holes 246 in the compressible member 218,as shown in FIGS. 17A-17D. The drill guide 300 then can be removed fromthe prosthetic device 200.

The anchor elements 310 can comprises rods, screws, or any othersuitable support structure. In addition, the anchor elements 310themselves can include small holes or irregularities on their surface,or they may have a bioreactive coating such as hydroxyapatite, toenhance fixation to the bone so that the anchor elements 310 do not backout of holes 246.

Although the figures show the anchor elements 310 arranged parallel toeach other, holes 246 can be arranged so that the anchor elements 310diverge or converge. Appropriate holes can be drilled through thecortical bone by reconfiguring drilling channels 308 in the drillpositioning block 306 to align with the converging/diverging holes inthe prosthetic device. Alternatively, the drill guide 300 may berepositioned after drilling a first hole through the bone that isaligned with a first hole in the prosthetic device, and before drillinga second hole that is aligned with a second hole in the prostheticdevice.

In the embodiment of FIGS. 17A-17D, the anchor elements 310 aredimensioned to traverse the entire diameter of the vertebral body toobtain bi-cortical purchase, that is, fixation in the hard outercortical bone on either side of the vertebral body. Alternatively, theanchor elements 310 can be shortened so that they to traverse only partof the diameter of the vertebral body to obtain uni-cortical purchase,that is, fixation in the hard outer cortical bone on only one side ofthe vertebral body.

Further, although the anchor elements 310 are shown passing through thebottom plate 244 of the compressible member 218, in alternativeembodiments, the site for placement of the anchor elements 310 can bethrough either plate 250, 252 of the fixation member 214 or through theupper plate 242 of the compressible member 218. Moreover, as shown inFIGS. 18A-18D, rather than passing through a portion of the prostheticdevice, the anchor elements 310 can pass under and adjacent thelowermost plate of the prosthetic device to minimize subsidence of thedevice into the cancellous subchondral bone in the central portion ofthe vertebral body.

More specifically, FIGS. 18A-18D show an embodiment of a prostheticdevice 400 having a fixation member 414 and a compressible member 418.The compressible member 418 has a bottom plate 444, here shown with aconvex lowermost surface 445. The convex lowermost surface 445 may beconfigured to sit within a correspondingly shaped concave seat 2050(shown in FIG. 24) formed in the cortical bone of an endplate of anadjacent vertebral body. The anchor elements 310 are positioned underand just adjacent to this lowermost surface 445 to minimize subsidenceof the prosthetic device 400.

In yet another aspect of the present invention, either or both of theplates of the fixation member can include a tab to help minimizesubsidence of the prosthetic device. FIGS. 19A-19E show a plate 500 fora fixation member that includes a tab 502 extending from the peripheryof the generally circular, main portion of the plate 500. The tab 502 isconfigured to rest on outer cortical bone of a vertebral body. In thisregard, FIG. 20 is a sectional view through a vertebral body,illustrating a plate 500 with a tab 502 extending into outer corticalbone 506 of the vertebral body 504. The tab 502 inhibits the ability ofthe prosthetic device to sink into the cancellous bone 508 of thevertebral body 504.

The plate 500 may also include a riser 510 through which holes 511,which are configured to receive anchor elements 310, can extend. Inaddition, a drill bit can be guided through the holes 511 in the riser510 to drill holes through the bone of the vertebral body, therebyeliminating the need for drill guide 300. Although plate 500 is shown inFIGS. 19A-19E and 22 with both a tab 502 and a riser 510 with holes 511for anchor elements 310, it will be understood that other embodiments ofthe plate can include only a tab, such as in FIG. 22A, or can includeholes to receive anchor elements 310 but lack a tab.

Another prosthetic device embodiment 700 that incorporates fixationplates like fixation plate 500 in embodiment shown in FIGS. 19A-19E isshown in FIGS. 21A-21D. This prosthetic device 700 has first and secondfixation members 714, 716 and a compressible member 818. The fixationmembers 714, 716 each include a fixation plate 750 on their outer ends;the plates 750 have tabs 751 extending therefrom. When the prostheticdevice 700 is implanted in upper and lower vertebral bodies, the tabs751 rest on the outer cortical bone of the vertebral bodies. Anchorelements 310 may extend through the cortical bone of the vertebralbodies and through holes 713 formed in the tabs 751, to enhance thestability of the prosthetic device 700.

In yet a further aspect of the present invention, the prosthetic devicecan include spacers 620 to maintain the fixation members 714 at adesired elongated position. The spacers 620 can be used in lieu of thelock screws 63 seen in FIG. 4 and their aforementioned alternatives.FIGS. 23A-23D show an embodiment of a spacer 620 that can be used withfixation members 714. The spacer 620 has a first box portion 622, asecond box portion 624, and a peg 626 projecting from the first boxportion 622.

In a prosthetic device employing a spacer 620 as shown in FIGS. 21A-21D, the inner plate 752 of the fixation member 714 can have a notch 754,and each plate 842, 844 of the compressible member can have a well 846.The peg 626 of the spacer 620 is configured to pass through the notch754 and snap into the well 846, to lock the spacer 620 in place in asnap fit, as shown in FIG. 21A. When the peg 626 of the spacer 620 islocked in the appropriate plate 842, 844, the second box portion 624resides between the adjustment elements 724 of the fixation member 714,as shown in FIG. 21B. The spacers 620 may be configured in varyingshapes and of various heights to accommodate different sized vertebrae.

The spacers 620 can be positioned in fixation members 714 after thefixation members 714 are properly positioned in the vertebral bodies.That is, once the fixation members 714 are positioned in the vertebralbodies, the tension or load experienced by the compressible member 818needs to be adjusted to optimize the normal loading and compression(i.e., the functionality) of the particular disc being replaced.

To optimize the normal loading and compression, a surgeon can use atensioner, (such as the tensioner described in U.S. Pat. No. 6,761,723)to move the plates 750, 752 of the fixation member 714 toward theendplates of the vertebral body. In this manner, the tensioner may beused to elongate the fixation member 714 until a proper elongationdistance between the plates 750, 752 is achieved. As the fixation member714 elongates, plate 752 contacts and encounters resistance from thecompressible member 818. The surgeon can continue to apply a load viathe tensioner to the fixation member 714 until a desired correspondingreactive load from the compressible member 818 is reached.

When the applied load measured by the tensioner equals the desired load,the surgeon knows that the fixation member 714 has been lengthened orelongated an appropriate amount to place the compressible member 818under the proper degree of tension. At this point, the spacer 620 can beinserted into the fixation member 714 until the peg 626 of the spacer620 snaps in the well 846. As a result, the spacer 60 maintains thefixation member 714 at the appropriate length. The tensioner then can beremoved from the fixation member 714.

As an alternative to the spacer 620, the struts 224 can be configuredfor adjustment like a crutch, that is, by having a hole through an outercasing and a plurality of holes through an adjustable inner member. Whenthe inner member is adjusted to the proper height, a fastener can beinserted through the hole in one side of the casing, through thecorresponding hole in the inner member, and then through the hole in theother side of the casing. The fastener immobilizes the inner member withrespect to the casing and maintains the proper elongation distancebetween the upper and lower plates 250, 252 of the fixation member 214.

Clamps also can be used to maintain the proper elongation distancebetween the plates 250, 252. The clamps can be C-shaped in cross sectionand have a length equal to the elongation distance. The C-shaped crosssection of the clamps leaves a slit or opening along their length. Theclamps also are resiliently flexible. When the slit of a clamp ispressed against a strut, the slit widens so that the clamp can be slidaround the strut. Once around the strut, the clamp returns to itsinitial shape. The clamps thus can be positioned on the struts 224 tosubstantially surround the struts 224 and maintain the proper elongationdistance between the plates 250, 252.

A tripod also can be used to maintain the proper distance between theplates 250, 252. The surgeon can select a tripod of an appropriateheight, that is, of a height equal to the desired elongation distance,and slide it into the fixation member 214. The surgeon then can positionthe legs of the tripod on the lower plate 252, preferably against threestruts 224, and position the top of the tripod against the upper plate250.

FIGS. 22A-22D show another embodiment of the prosthetic device 700′.This prosthetic device 700′ is similar to the prosthetic device 700 ofFIGS. 21A-21D, except that the outer plates 750′ of the fixation members714′ of the device 700′ shown in FIGS. 22A-22D do not include the risers753 present on the outer plates 750 of the fixation members 714 of thedevice 700 shown in FIGS. 21A-21D. All other elements of the fixationmembers 714′, 716′ of the device 700′ shown in FIGS. 22A-22D are thesame as the corresponding elements of the fixation members 714, 716 ofthe device 700 shown in FIGS. 21A-21D and, therefore, are similarlynumbered but include a differentiating apostrophe.

Another prosthetic device embodiment, which is shown in FIG. 24,restores normal biomechanics and motion of a pair of failing, adjacentintervertebral discs. The prosthetic device 2000 is designed to sparethe annulus fibrosis of the discs and the anterior longitudinal ligamentof the spine. Moreover, the prosthetic device ensures solid bonefixation via attachment to cancellous vertebral body bone, rather thanto the external surface of non-uniform and/or sclerotic vertebral bodyendplates.

The prosthetic device 2000 may be implanted and adjusted in a procedurethat should not take longer than current spinal fusion procedures.Further, it will be understood from the following description that inconjunction with the previously described embodiments, due to itssomewhat modular construction, the prosthetic device can be modified toreplace more than two intervertebral discs by including an appropriatenumber of fixation members and compressible members, as needed. Arelated embodiment of the invention (later discussed in detail)addresses a method of spinal prosthetic implantation by which one ormore intervertebral discs may be replaced by an implantable prostheticdevice.

The prosthetic device 2000 includes first and second compressiblemembers 2020 and a fixation member 2030 sized to fit within a cavity inthe vertebral body 2130 between the first compressible member 2020 andthe second compressible member 2020. The first compressible member 2020is sized to substantially replace the nucleus pulposus of a firstintervertebral disc 2120. Similarly, the second compressible member 2020is sized to substantially replace the nucleus pulposus of a secondintervertebral disc 2140 that is separated from the first intervertebraldisc 2120 by the vertebral body 2130. In the embodiment shown, thesecond compressible member 2020 has the same structure as the firstcompressible member 2020.

Each compressible member 2020 comprises a compressible body portionformed of one or more compressible bodies 2022. The compressible bodies2022 may be made of a biocompatible material compressible in an axialdirection (i.e., in a direction substantially parallel to the spine).

The compressible members 2020 include a first plate 2024 proximal to thefixation member 2030 and a second plate 2026 distal from the fixationmember. The second plates 2026 may have sections 2027 having convexsurfaces 2028, which may serve as and function like the second fixationmember shown in FIG. 7. Further, as later explained in detail, theconvex surfaces 2028 may be sized to sit within correspondingly shapedconcave seats 2050 formed in the cortical bone endplates of vertebralbodies.

The fixation member 2030 may include one or more adjustment members 2038and/or a locking mechanism, as best shown in FIGS. 26 and 27. Theadjustment members 2038 may be telescoping struts, the length of whichmay be fixed by a locking mechanism such as the c-shaped clamps,tripods, spacers, or other suitable extension devices, as previouslydiscussed. By fixing the height of the fixation member 2030, the lockingmechanism fixes the load applied to the fixation member 2030.

Similar to the embodiments previously discussed with respect to FIGS.15-21, the fixation member 2030 additionally comprises a first plate2032 and a second plate 2034. The first and second plates 2032, 2034 areconfigured to engage the first plates 2024 of the compressible members2020, in a dovetail tenon/mortise relationship. Specifically, as shownin FIGS. 24 and 25, the first plates 2024 have projections 2027 whichare sized to engage correspondingly shaped slots 2039 formed in thefirst and second plates 2032, 2034 of the fixation member 2030. Thefixation member 2030 engages the compressible members 2020 by slidingthe slots 2039 formed in the first and second plates 2032, 2034 of thefixation member 2030 onto the adjacent correspondingly shapedprojections 2027 formed on the first plate 2024 of the compressiblemembers 2020.

If only one intervertebral disc needs to be replaced, the second plate2034 of the fixation device 2030 can rest against an interior side ofthe subchondral bone of the endplate of a vertebral body adjacent thefailing disc. In this instance, the fixation member 2030 would bepositioned in a vertebral body adjacent the failing disc and thecompressible member 2020 would be positioned in a failing disc. Thesecond plate 2026 of the compressible member 2020 may sit within a seat2050 formed in the cortical bone of the vertebral body above the disc.

Similar to the aforementioned embodiments, to reduce the risk ofsubsidence, the compressible members 2020 may include drilling channels2029 through the second plates 2026. The drilling channels 2029 may beconfigured to receive anchor elements 2160 (e.g., screws, otherfasteners, plates, etc.), as shown in FIG. 28. The anchor elements 2160may be journalled through the drilling channels 2029 and into thecortical bone of a vertebral body, in the manner previously discussed.As a result, the orientation of the compressible members 2020 withrespect to the vertebral body may be additionally stabilized. Inaddition, either plate of the fixation member 2030 and either plate ofthe compressible members 2020 can include a tab, as previouslydiscussed, to help minimize subsidence of the prosthetic device.

To implant the prosthetic device 2000, a cavity is created in thevertebral body between the two discs to be reconstructed, in the mannerpreviously described. This procedure, which involves excising bonematter from the vertebral body, can be performed using a cutting guide,a chisel and a chisel guide, and a reamer (such as those described inU.S. Pat. No. 6,761,723) and/or using surgical implements discussedherein with respect to FIGS. 33-36. Bone harvested from the vertebralbody by the reamer can be used after implantation of the prostheticdevice to promote bone ingrowth into the prosthetic device, aspreviously described. This procedure creates a cavity bounded bysubchondral bone of the endplates of the vertebral body.

Once the cavity in the vertebral body is formed, an endplate and nucleuscutter attached, for example, to a-distractor 920 (shown in FIG. 34B),may be used to cut (which may be in the form of boring) through thefirst endplate of the vertebral body adjacent the first failingintervertebral disc to be replaced. Once through the endplate, thecutting can continue through the nucleus pulposus of the first failingdisc to excise the nucleus pulposus thereof, creating a cavity for acompressible member 2020. In this method, the annulus fibrosis ismaintained, although it is envisioned that, if the cutting inadvertentlycuts into an inner portion of the annulus fibrosis, the annulus fibrosisstill may be capable of securely retaining the compressible member 2020.Additional cutting may be performed into the endplate of the vertebralbody on the other side of the first failing intervertebral disc, therebyforming a seat 2050 against which a convex surface 2028 of thecompressible member 2020 may be positioned. The seat 2050 will begenerally concave in shape so as to better engage the convex surface2028 of the compressible member 2020.

Once the nucleus pulposus of the first failing intervertebral disc hasbeen excised and the seat 2050 has been formed, the same process may beused to bore through the other endplate of the vertebral body in whichthe cavity was formed and through the nucleus pulposus of the secondfailing disc adjacent it. Further, a seat 2050 may be formed in thevertebral body on the other side of the second failing disc.

Once cutting is completed, a first compressible member 2020 ispositioned in the cavity in the vertebral body and then pushed throughthe hole in the endplate and into the space left by the excised nucleusof the first failing disc.

As the first compressible member 2020 is inserted into the first failingdisc, the convex surface 2028 is pushed into the seat 2050 in thevertebral body endplate on the other side of the disc. Although theconvex surface 2028 may be positioned in cancellous bone in the interiorof a vertebral body, it is preferably position in the cortical bone, tohelp minimize the risk that the second plate 2026 will, over time,undesirably creep into the vertebral body as a result of loading.

Once the first compressible member 2020 is fully implanted, a secondcompressible member 2020 is inserted into the second failingintervertebral disc in the same manner. Similarly, the convex surface2028 of the second compressible member 2020 is inserted into the seat2050 adjacent the second failing disc. It should be readily recognizedthat the order in which the compressible members 2020 are inserted canbe reversed.

Once the compressible members 2020 are in place, the surgeon slides afixation member 2030 into the cavity in the vertebral body whileengaging the projections 2027 of the compressible members 2020 and theslots 2039 in the first and second plates 2032, 2034 of the fixationmember 2030. As a result, the fixation member 2030 is fixedly joined tothe compressible members 2020. In other embodiments, the compressiblemembers 2020 and the fixation member 2030 may be connected byconventional attachment members, such as screws, or by biocompatiblecement or a suitable adhesive composition.

When the fixation member 2030 is in place, the length of the adjustmentmembers 2038 can be adjusted to fix the length of the fixation member2030, in the manner previously described. Similarly, the length of thefixation member 2030 can be maintained by using a locking mechanism,such as spacer 2036, which prevents further inward adjustment of theadjustment members 2038, as previously described. Alternatively,fixation members can be available in a variety of fixed sizes; aproperly sized fixation member could be selected for implantation in thecavity of the vertebral body, thereby negating the need for a lockingmechanism. Regardless, when the length of the fixation member 2030 isfixed, the cavity in the vertebral body may be filled with bone graft,as previously described.

To reduce the risk of subsidence, the compressible members 2020 and/orthe fixation member 2030 may have drilling channels 2029 for receivinganchor elements 2160 (e.g., screws) to supplement immediate fixationduring healing of the bone graft, as previously described.

A porous bone ingrowth coating and/or surface texturing may also beapplied to the device. For example, hydroxyapatite or otherbone-to-implant chemical or biological interface surface treatment maybe applied to the first and second plates 2032, 2034 of the fixationmember 2030 and/or to the convex surfaces 2028 (each of which is incontact with bone), to enhance bone growth into a textured poroussurface.

If either of the vertebral bodies adjacent the compressible members 20are scoliotic, the compressible elements 2022 used in the compressiblemembers 2020 may be designed to combat this problem. Specifically, theselected compressible elements 2022 may have spring constants which aregreater or less than the spring constants of the remaining compressibleelements 2022. As a result, corrective loading on the scoliotic bodiescan be better achieved.

Unlike conventional implanted prosthetic devices, which are typicallynot recommended for replacing two discs, this prosthetic deviceembodiment 2000 easily replaces the nucleus pulposuses of theintervertebral discs both above and below a vertebral body. Further, notonly is this approach more efficient for surgeons, it avoids theproblems inherent in separately distracting two disc spaces sufficientlyto insert total disc replacements.

Various intervertebral disc prosthetic device embodiments (and themethods of implanting them) have been described. In conjunction withthese embodiments, however, various modifications may be used to addressa particular patient's condition and/or the level in the spine in whichthe device will be implanted (e.g., between Lumbar-5 and Sacrum-1 thereis a great variation among patients in the shape of the joint).Accordingly, the following describes various alternative compressiblemember embodiments which may be employed with any of the aforementionedprosthetic device embodiments.

FIGS. 29A-29E show two alternate embodiments for a compressible member.Specifically, FIGS. 29A and 29B respectively show a cross-sectional viewand a perspective view of a compressible member 2100 and FIG. 29E showsthe compressible member 2100 positioned in a disc.

The compressible member 2100 is formed of a base member 2102 that may,as shown, be in the shape of a cup. The lower surface 2112 of the basemember 2102 may be attached to a fixation member (not shown in FIGS.29A-29E) in any manner previously discussed (e.g., screws, dovetailtenon/mortise joint, etc.)

A circumferential wall 2114 of the base member 2102, which rises upwardfrom the lower surface 2112, encloses a plurality of compressibleelements 2104 (e.g., springs, or any other compressible elementpreviously discussed). As shown in FIG. 29E, the wall 2114, whenimplanted, compensates for anatomic variations and assures that theendplate 2122 of the vertebral body 2120 engages solid metal.

The other ends of the compressible elements 2104 are attached to anupper member 2106, in a manner similar to the previously describedcompressible member embodiments. Similarly, the upper member 2106 mayhave a convex surface 2108 which is configured to rest within a seat2050 (shown in FIG. 24) formed in a vertebral body endplate, aspreviously discussed.

The compressible member embodiment 2100′ shown in FIGS. 29C-29D issubstantially similar to the compressible member embodiment 2100 shownin FIGS. 29A-29B, except that the wall 2114 in the base member 2102 inthe embodiment shown in FIGS. 29A-29B is replaced with a slotted wall2114′ defining an alternative base member 2102′.

The reason for the slotted wall 2114′ is that for some individualsand/or some disc locations, additional clearance may serve to facilitateplacing the springs over as wide an area as possible. However, as theslots reduce the support and attachment to the cortical bone of thevertebral body, the desires to use certain spring designs and to enhancesupport/attachment must be weighed in each particular instance.

Another compressible member 2200 embodiment is shown in FIGS. 30A-30B.The compressible member 2200 includes a base member 2202, compressibleelements 2204, and an upper member 2206. In this embodiment, the basemember 2202 is wider than the diameter of the hole 2210 but narrowenough so that it can go through the hole 2210 at an angle after whichit can be maneuvered so as to cover the hole 2210, as shown in FIG. 30B.In either case, the base member 2202 rests against the cortical bone ofthe vertebral body 2220, thereby reducing the likelihood that thecompressible member 2200 may experience subsidence into the vertebralbody 2220 as a result of cyclical loads applied to the compressiblemember 2200.

Another compressible member 2200′ embodiment is shown in FIGS. 30C and30D. In this embodiment, the base member 2202′ is expandable to be widerthan the diameter of the hole 2210 in a vertebral body endplate throughwhich the compressible member 2200′ is implanted. Specifically, the basemember 2202′ includes a rotatable driving plate 2240 and a plurality ofradially adjustable leaves 2250. The rotatable plate includes aplurality of projections 2242 that, when the rotatable plate 2240rotates, push the leaves 2250 radially outward along rails 2260, therebyradially adjusting the overall diameter of the base member 2202′. As aresult, after the compressible member 2200′ is pushed through the hole2210, the base member 2202′ may be radially expanded to fix the basemember 2202′ in a manner similar to that shown in FIG. 30B. In addition,although the size of base member 2202′ is described as being adjusted bymeans of leaves, the embodiment is not so limited. Rather, the basemember could be adjusted in other ways such as, for example, by means ofscrews, telescoping rods, etc.

Another compressible member 2400 embodiment is shown in FIG. 31. A firstcompressible member 2300, which is provided in a first disc, abuts(along a concave seat 2050) a first vertebral body 2430 and is connectedto a fixation member 2440. This first compressible member 2300 and thefixation member 2440 may be any of the compressible member embodimentsand fixation member embodiments, respectively, previously discussed.However, it is a second compressible member 2400, which is alsoconnected to the fixation member 2440, which is the focus of FIG. 31.Moreover, although the second compressible member 2400 is shown as beingpart of a dual compressible member device, it should readily berecognized that it could be incorporated in a single compressible memberdevice.

The second compressible member 2400, like previous embodiments, includesa base member 2402 supporting a plurality of compressible elements 2404.The other ends of the compressible elements 2404 are connected toanother plate 2406. Whereas in previous embodiments, the plate 2406would rest against a vertebral body, in this embodiment, the plate 2406is attached to a ball-and-socket joint comprised of a socket 2408 and aball 2410. The socket 2408 is attached to the plate 2406 and the ball2410 is immobilized in the vertebral body 2460 by means of a spike 2412or screw.

The purpose of the ball-and-socket connection is to accommodate anatomicvariation in which the angle between vertebral endplates may be highlyvariable among patients. This is particularly helpful between Lumbar-5and Sacrum-1 where there is a great variation among patients in theshape of the joint and where replacement of two discs (as shown in FIG.31) in this area is particularly complicated.

It should be readily recognized that the compressible member embodiments2300, 2400 shown in FIG. 31 can be switched. In other words, thecompressible member embodiment 2300 currently adjacent the uppervertebral body 2430 can be switched with the compressible memberembodiment 2400 currently adjacent the lower vertebral body 2460.

FIG. 32 shows another embodiment of a compressible member 2500, whichlike the embodiment 2400 shown in FIG. 31 employs a spike 2512 toimmobilize a plate 2506, as hereafter explained in detail. Thecompressible member embodiment 2500, like previous embodiments, has abase member 2502, an upper member 2506, and a plurality of compressibleelements 2504 which extend between the base member 2502 and the uppermember 2506.

Whereas in many of the previously described embodiments, the end of acompressible member away from a fixation member was formed to have aconvex surface configured to engage a concave seat 2050 formed in avertebral body, in some instances such an engagement may not provideadequate support for the compressible member. As a result, in thecompressible member embodiment 2500 shown in FIG. 32, the plate 2506adjacent the vertebral body 2520 is formed with a spike 2512 that isconfigured to penetrate into the vertebral body 2520 so as to immobilizethe plate 2506 with respect to the vertebral body 2520. Moreover, thelikelihood of subsidence of the compressible member 2500 into thevertebral body 2520 is slight as a result of the remainder of the plate2506 abutting the cortical bone endplate of the vertebral body 2520.

The aforementioned described various implantable prosthetic devices andthe methods by which they may be implanted. In conjunction with thesedevices and their methods of implantation, this invention also addressesvarious tools by which the implantation methods may be performed.

FIGS. 33A-33C illustrate a surgical implement, that is, a cuttingimplement that can be mounted to a compressor 900 (shown in FIG. 34A) ora distractor 910 (shown in FIG. 34B) to cut through an endplate of avertebral body and the nucleus pulposus of the intervertebral discadjacent the vertebral body. The exemplary cutting implement is in theform of an endplate and nucleus cutter 920 having a substantiallycircular sidewall 921 that terminates in a cutting edge 922.

The maximum diameter of the sidewall 921 of the endplate and nucleuscutter 920 should not be greater than the minimum diameter of thenucleus pulposus and/or the diameter of the prosthesis to be implanted.In addition, the cutting edge 922 can be smooth or, alternatively,serrated. The cutting edge 922 may be thinner than the sidewall 921 andmay be tapered to a sharp end. The endplate and nucleus cutter 920optionally can have a projection 923, as shown in FIG. 33B. The tip ofthe projection 923 can be used to create a notch in an endplate, therebybracing the endplate and nucleus cutter 920 relative to the endplate;the projection 923 can serve as an axis of rotation. Moreover, thisbracing effect enables a surgeon to cut through the endplate with thesharp end of the endplate and nucleus cutter 920, without risk that theendplate and nucleus cutter 920 will inadvertently slide from its properposition relative to the endplate surface.

An alternative embodiment of the endplate and nucleus cutter 920′ isshown in FIG. 33C. The only difference between this embodiment and theone shown in FIG. 33B is that the projection 923′ is cylindrical inshape and has a concave end. An advantage of employing the embodiment ofFIG. 33C with the embodiment of FIG. 33B on a single compressor 900 isthat when the sharp edges of the two endplate and nucleus cutters 920approach each other, the tip of the projection 923 on the first cutter920 will be partially journalled into the concave end portion of theprojection 923′ of the second cutter 920′.

As shown in FIG. 34A, an endplate and nucleus cutter 920 can be attachedto an end portion 901 of a first arm 902 of the compressor 900 to face asecond arm 904. Similarly, an endplate and nucleus cutter 920, which isattached to an end portion 903 of the second arm 904, faces toward thefirst arm 902 and toward the other endplate and nucleus cutter 920.

When the handle 905 of the compressor 900 is compressed, the first andsecond arms 902, 904 move toward each other. In addition, as the firstand second arms 902, 904 move toward each other, they maintain theirapproximately parallel orientation, and the endplate and nucleus cutters920 approach each other. The endplate and nucleus cutters 920 on thefirst and second arms 902, 904 can share a common central axis so that,when the handle 905 is fully compressed, the cutting edges 922 of theendplate and nucleus cutters 920 contact each other.

The endplate and nucleus cutters 920 can be either fixedly mounted orrotatably mounted to the arms 902, 904 of the compressor 900. When theendplate and nucleus cutters 920 are fixedly mounted, the surgeon canmanually rotate the cutters 920 by swinging the handle 905 of thecompressor 900 side-to-side. This side-to-side motion, combined withcompression of the handle 905, enables the cutting edges 922 to cutthrough the endplate and nucleus pulposus of the damaged disc.Alternatively, the endplate and nucleus cutters 920 may be rotatablymounted to the compressor 900. A motor or other drive source can beconnected to the cutters 920 to rotate them relative to the arms 902,904 of the compressor 900.

The compressor 900 can be used when a surgeon wants to implant aprosthetic device having two fixation members, one of which is to gointo a vertebral body above a problematic disc and the other of which isto go into the vertebral body below the problematic disc. FIG. 35 showsa compressor 900 being inserted into adjacent vertebral bodies to removethe nucleus pulposus of a damaged disc.

In some situations, however, the surgeon needs to implant only onefixation member (e.g., the embodiments shown in FIGS. 15-18 and 24-28)or only one non-extendable fixation member coupled to a compressiblemember (e.g., the embodiment shown in FIG. 7). In such situations, adistractor 910 with only one, outwardly facing endplate and nucleuscutter 920 may be used.

FIG. 34B shows a distractor 910 having one endplate and nucleus cutter920 on a first arm 912 which faces outward and away from a second arm914. An outwardly facing plate 930 is rotatably attached to the secondarm 914 by an axle 931. The plate 930 is designed to be placed againstan endplate in a vertebral body and to remain immobile relative to thevertebral body.

As the endplate and nucleus cutter 920 of the distractor 910 either ismanually rotated by the surgeon (in an embodiment where the endplate andnucleus cutter 920 is fixedly mounted to the distractor 500) or rotatesas a result of a motor (in an embodiment where the endplate and nucleuscutter 920 is rotatably mounted to the distractor 910), the endplate andnucleus cutter 920 will cut through one endplate in a vertebral body,while the plate 930 remains pressed against the other endplate in thevertebral body. The plate 930 will not abrade the vertebral body againstwhich it is placed because it does not rotate with respect to thatendplate.

When the arms 912, 914 of the distractor 910 are inserted into a cavityin a vertebral body and the handle 915 is subsequently compressed, theplate 930 will move in one direction to contact the endplate of thevertebral body, and the endplate and nucleus cutter 920 will move in anopposite direction to contact the other endplate of the vertebral body.Continued compression of the handle 915 and rotation of the endplate andnucleus cutter 920 will force the cutter 920 through the endplate andthe nucleus pulposus of the adjacent intervertebral disc.

It will be understood that an endplate and nucleus cutter 920 can bemounted to devices having a configuration different than the compressor900 and distractor 910. For example, an endplate and nucleus cutter 920can be attached to an end of a single arm, and a surgeon can grip theopposite end of the single arm to position the endplate and nucleuscutter 920 appropriately to cut through the endplate and the nucleuspulposus of a damaged disc. The single arm can be bent to provideadditional leverage.

FIGS. 36A-36D illustrate another embodiment of an endplate and nucleuscutter 1000. This cutter 1000 includes a rotating axle 1002 withmultiple arms 1004, a cylindrical main body 1006 with a pair of obliqueslots 1008 to receive the arms 1004 of the axle 1002, and a cuttingsurface 1010 that attaches to the cylindrical main body 1006. Thecutting surface 1010 can have a flat profile or it can have a convex,domed profile as seen in FIGS. 36A, 36C, and 36D. The cutting surface1010 includes cutting edges 1012 that enable the cutter 1000, whenrotated, to cut through the endplate and the nucleus pulposus of thedisc.

In another embodiment shown in FIG. 36E, a serrated cutting edge 1010′can be defined around a perimeter of a cup-shaped cutter 1000′ which issimilar in shaped to the endplate and nucleus cutter 920 shown in FIGS.33A and 33B.

The cutter 1000 can be mounted to the arm of a compressor or adistractor and, once positioned at a cutting location in a vertebralbody, can elongate and move away from the arm. Accordingly, the cutter1000 can be placed through a relatively small vertebral body window andstill reach all the way through the vertebral body endplate and thenucleus pulposus of the damaged disc. When the cutter 1000 is in theshortened position, the arms 1004 of the axle 1002 are positioned in theslots 1008 at a location close to the cutting surface 1010. Withrotation of the axle 1002, friction forces the arms 1004 to slide up theslots 1008, which in turn elongates the cutter 1000 and moves thecutting surface 1010 toward the area to be cut.

The intervertebral prosthetic device embodiments of the presentinvention offer several advantages. For example, the intervertebralprosthetic device embodiments replicate the mechanical properties of anatural intervertebral disc. The intervertebral prosthetic deviceembodiments restore disc height, defined as the axial distance betweenvertebrae adjacent the damaged disc, and duplicate the range of motionof a natural intervertebral joint.

As the prosthetic device embodiments have no ball bearings (with theexception of the ball-and-socket joint of the embodiments shown in FIG.31), rollers, or hinges, the intervertebral prosthetic deviceembodiments suffer minimal degradation of the prosthetic material andproduce minimal wear debris under long-term cyclic loading conditions.Further, the prosthetic device embodiments: (a) can axially compress andthus dissipate energy; (b) may be easily repaired or replaced; (c) maybe easily manufactured and implanted by a surgeon; and (d) are durableand modular. Moreover, as the prosthetic device embodiments need notinclude plastic polymers or elastomeric components, the prostheticdevice embodiments do not degrade under long-term cyclic loadingconditions.

It should be understood that the benefit of the implantation procedurefor the one compressible member/one fixation member embodiment and thedual compressible member/one fixation member embodiments is that onlyone vertebral body cavity is formed. As a result, both the timenecessary for the implantation procedure and the amount of resultanthealing are greatly reduced.

Although the previously described embodiments of the intervertebralprosthetic device include an adjustable fixation member, it will beunderstood that the intervertebral prosthetic device can include a rigidfixation member sized specifically to fit the vertebral body and toadequately pretension the compressible member. Rigid fixation memberscan be made in various sizes so that a surgeon can select anappropriately sized fixation member for the particular surgical site.

The prosthetic device embodiments can comprise biocompatible metallicmaterials, such as a titanium alloy having, for example, 4% vanadium and6% aluminum. Persons of skill in the art will recognize other suitablematerials, for example, a cobalt-chromium alloy, such as alloy number301. Alternatively, the prosthetic device embodiments, with theexception of the springs of the compressible member, can comprise aceramic material, such as aluminum oxide or zirconium oxide. The poroussurfaces of the fixation members can be coated with hydroxyapatite orbioactive proteins (e.g., bone morphogenic protein) to encourage boneingrowth.

The fixation members of the prosthetic device embodiments, which may becomposed of carbon fiber polyetheretherketone, bone graft (auto- orallo-graft bone), bone cement, etc., support the compressible member(s)until the bone graft (which is packed into the open space of thefixation members) heals. Once the bone graft heals, however, thefixation members may no longer be needed. Accordingly, the fixationmembers of the prosthetic device embodiments may be composed of abioresorbable material that would gradually be replaced by bone overtime. Suitable bioresorbable materials to form the fixation membersinclude structural allograft (bank) bone, or polymers made of polylacticacid or polyglycolic acid. Similarly, the anchor members also can bemade of carbon fiber or of a bioresorbable material, such as polylacticacid, polyglycolic acid, or a combination of those materials.

The compressible members may be, for example, springs, elastomers,monolithic bodies, elastic polymers, hydrogels, disc allograft, or anyother material which displays similar mechanical properties when placedunder stress (i.e., tension and/or compression) and which substantiallyregains its original shape upon removal of the stress.

The embodiments of the prosthetic device previously described haveadvantages over conventional devices. For example, although theprosthetic device may be implanted using a straight anterior approach,it may be implanted using an anterolateral approach to the spine that isa retroperitoneal approach in the plane between the abdominal vesselsand the psoas muscle.

Unlike the conventional and more dangerous straight anterior approachrequired by total disc replacement devices (which sever the anteriorlongitudinal ligament and/or sever the annulus fibrosis, both of whichdisrupt tissues that will not heal), the embodiments described hereinonly disrupt bone material in the adjacent vertebral body and thenucleus pulposus in the intervertebral disc. The bone heals and thenucleus pulposus is replaced by the prosthetic device.

Further, the embodiments of the prosthetic device described hereinminimally infringe upon areas of the vertebral body which would be usedto provide a fusion should that later become necessary. Specifically, bymaintaining the anterior longitudinal ligament, the anterolateralapproach helps maintain spinal function and stability. In addition, ananterolateral approach on one side of a vertebral body allows for alater opposite side approach for adjustment of the device or foradjacent level disc replacement should that become necessary. Further,this opposite side approach would not be hindered by scar tissue fromthe previous procedure.

The prosthetic device embodiments also allow for bending and torsionmotion, as well as axial displacement and elastic compression. Further,unlike an articulated joint, the prosthetic device deforms similarly toa normal, healthy disc. Moreover, unlike previous total disc replacementdevices which may result in motion of about 3.8° to 4.6°, theembodiments of the invention herein described maintain the motion at anearly healthy level of motion, i.e., about 7° to about 12°.

The ability to pretension the fixation member allows for a more preciserestoration of disc height. Further, as the fixation members may beanchored entirely to cancellous bone, at least some embodiments avoidproblems inherent to poor bony ingrowth, which may result from scleroticendplates. As a result, the risk of device loosening is minimized. And,as the device is enclosed entirely by bone and the annulus fibrosis,ejection, dislocation, and migration of the device is very unlikely. Inaddition, intramedullary fixation of the fixation member in thevertebral body provides greater stability. The fixation member isprovided within the cancellous bone of a vertebral body adjacent thefailing disc(s), to maximize the osteogenic potential of bone to growinto the fixation member. Further, the replacement of the autologousbone removed from the vertebral body during the procedure (or theaddition of bone cement) into the open vertebral body facilitates thetransfer of loads to the cortical bone walls of the vertebral body oncethe bone heals.

Moreover, fibrous soft tissue growth into the compressible members willfill the normal volume of the disc nucleus. As a result, as thecompressible member is compressed, this tissue will bulge outward andradially load the inner annulus fibrosis in a manner similar to ahealthy nucleus. The radially outward loading will restore the functionof the retained annulus fibrosis.

Finally, the prosthetic device embodiments may be used no matter howcollapsed a patient's disc may be. An overdistraction problem inherentfor installation of total disc replacements does not arise with respectto the prosthetic device described herein.

The preferred embodiments have been set forth herein for the purpose ofillustration. This description, however, should not be deemed to be alimitation on the scope of the invention. Additional advantages andmodifications will readily occur to those skilled in the art. Therefore,the invention in its broader aspects is not limited to the specificdetails, and representative devices, shown and described herein.Accordingly, various modifications may be made without departing fromthe spirit or scope of the general inventive concept as defined by theappended claims and their equivalents.

1. A prosthetic device comprising: a first compressible member sized tosubstantially replace the nucleus pulposus of a first intervertebraldisc; a second compressible member sized to substantially replace thenucleus pulposus of a second intervertebral disc that is separated fromthe first intervertebral disc by a vertebral body; and a fixation membersized to fit within a cavity in the vertebral body between the first andsecond compressible members.
 2. The prosthetic device according to claim1, wherein the fixation member comprises: at least one adjustable memberconfigured to adjust a length of the fixation member; and a lockingmechanism configured to maintain the adjustable member at a fixedlength.
 3. The prosthetic device according to claim 2, wherein thelocking mechanism is a spacer.
 4. The prosthetic device according toclaim 1, wherein each of the first and second compressible memberscomprises at least one of a spring, an elastomer, and disc allograft. 5.The prosthetic device according to claim 1, wherein a plate of each ofthe first and second compressible members is configured to be positionedwithin a seat formed in an endplate of the vertebral body adjacentthereto.
 6. A prosthetic device comprising: a fixation member sized tofit within a cavity in a first vertebral body; and a compressible membersized to substantially replace a nucleus pulposus of an intervertebraldisc adjacent the vertebral body, wherein: (A) a first side of thecompressible member is configured to engage the fixation member and asecond side of the compressible member is configured to engage a secondvertebral body, and (B) the second side of the compressible member isconfigured to fit within a seat formed in the cortical bone of theendplate of the second vertebral body.
 7. An intervertebral prostheticdevice for implantation in a spine, comprising: a rigid fixation memberhaving a fixed length, the rigid fixation member being configured to beplaced in a cavity of a vertebral body and against bone of the vertebralbody; and a first compressible member configured to be placed in acavity in a first intervertebral disc adjacent the vertebral body and tobe secured to the rigid fixation member, wherein: (A) the compressiblemember is constructed to remain compressible after implantation and hasat least one compressible element that remains compressible afterimplantation, and (B) the rigid fixation member is sized to compress thecompressible member a predetermined amount when the rigid fixationmember and the first compressible member are placed in the cavity in thefirst vertebral body and in the cavity in the first intervertebral disc,respectively.
 8. The intervertebral prosthetic device according to claim7, further comprising: a second compressible member, wherein the secondcompressible member is configured to be: (A) placed in a cavity in asecond intervertebral disc adjacent the vertebral body on the oppositeside as the first intervertebral disc; and (B) secured to the rigidfixation member.
 9. The intervertebral prosthetic device according toclaim 8, wherein the first and second compressible members are sized toreplace the nucleus pulposuses of the first and second intervertebraldiscs, respectively.
 10. The intervertebral prosthetic device accordingto claim 7, wherein the rigid fixation member is composed of abioresorbable material.
 11. The intervertebral prosthetic deviceaccording to claim 7, wherein the first compressible member has a plateopposite the rigid fixation member having a convex outer surface. 12.The intervertebral prosthetic device according to claim 11, wherein theconvex lowermost surface is configured to rest within a correspondingconcave indentation formed in a second vertebral body adjacent thereto.13. An intervertebral prosthetic device for implantation in a spinecomprising: a fixation member configured to be placed in a cavity of avertebral body, the fixation member comprising: an outer memberconfigured to be placed against bone of the vertebral body; an innermember opposite the outer member; and at least one adjustment elementthat extends between the outer and inner members and that is configuredto adjust a length dimension of the fixation member along itslongitudinal axis; a compressible member configured to be placed in acavity in an intervertebral disc adjacent the vertebral body andconfigured to be secured to the inner member of the fixation member; anda spacer sized to fit between the outer and inner members of thefixation member to maintain the fixation member at a desired lengthdimension.
 14. The intervertebral prosthetic device according to claim13, wherein the outer and inner members of the fixation member compriseouter and inner plates, respectively.
 15. The intervertebral prostheticdevice according to claim 14, wherein the inner plate of the fixationmember includes a notch through which a portion of the spacer extends.16. The intervertebral prosthetic device according to claim 15, whereinthe portion comprises a peg.
 17. The intervertebral prosthetic deviceaccording to claim 13, wherein the spacer includes a peg.
 18. Theintervertebral prosthetic device according to claim 17, wherein thecompressible member includes a well dimensioned to receive the peg in asnap fit.
 19. The intervertebral prosthetic device according to claim13, wherein the compressible member has a bottom plate with a convexlowermost surface.
 20. The intervertebral prosthetic device according toclaim 19, wherein the convex lowermost surface is configured to restwithin a corresponding concave indentation formed in a second vertebralbody adjacent thereto.
 21. An intervertebral prosthetic device forimplantation in a spine, comprising: a fixation member configured to beplaced in a cavity of a vertebral body, the fixation member comprising:an outer member configured to be placed against bone of the vertebralbody; an inner member opposite the outer member; and a longitudinal axisextending between the outer and inner members; and a compressible memberconfigured to be placed in a cavity in an intervertebral disc and to besecured to the inner member of the fixation member, wherein: (A) thecompressible member is constructed to remain compressible afterimplantation, and (B) the outer member includes a tab extending outwardalong an axis different from the longitudinal axis.
 22. Theintervertebral prosthetic device according to claim 21, wherein theouter and inner members of the fixation member comprise outer and innerplates, respectively.
 23. The intervertebral prosthetic device accordingto claim 21, wherein the compressible member has a bottom plate with aconvex lowermost surface.
 24. The intervertebral prosthetic deviceaccording to claim 23, wherein the convex lowermost surface isconfigured to rest within a corresponding concave indentation formed ina second vertebral body adjacent thereto.
 25. An intervertebralprosthetic device for implantation in a spine, comprising: a fixationmember configured to be placed in a cavity of a vertebral body, thefixation member comprising: an outer member configured to be placedagainst bone of the vertebral body; an inner member opposite the outermember; and a longitudinal axis extending between the outer and innermembers; a compressible member configured to be placed in a cavity in anintervertebral disc adjacent the vertebral body and configured to besecured to the fixation member; and at least one anchor elementconfigured to immobilize and/or stabilize the compressible member and/orthe fixation member.
 26. The intervertebral prosthetic device accordingto claim 25, wherein compressible member is constructed to remaincompressible after implantation and has at least one compressibleelement that remains compressible after implantation.
 27. Theintervertebral prosthetic device according to claim 25, wherein theanchor element is dimensioned to extend into at least one of thefixation member and the compressible member and through bone adjacent toat least one of the fixation member and the compressible member.
 28. Anintervertebral prosthetic device according to claim 25, wherein theanchor element extends through bone adjacent the outer member of thefixation member and into the outer member.
 29. The intervertebralprosthetic device according to claim 25, wherein the compressible memberhas a bottom plate with a convex lowermost surface.
 30. Theintervertebral prosthetic device according to claim 29, wherein theconvex lowermost surface is configured to rest within a correspondingconcave indentation formed in a second vertebral body adjacent thereto.31. A prosthetic device comprising: a fixation member sized to fitwithin a cavity in a first vertebral body; and a compressible membercomprising: a cup-shaped base member; an upper member; and one or morecompressible elements provided between the base member and the uppermember, wherein the compressible member is sized to substantiallyreplace a nucleus pulposus of an intervertebral disc adjacent thevertebral body.
 32. The prosthetic device according to claim 31, whereinthe base member of the compressible member is configured to engage thefixation member and the upper member of the compressible member isconfigured to engage a second vertebral body.
 33. The prosthetic deviceaccording to claim 31, wherein the cup-shaped base member has a wall,and wherein one or more slots are provided in the wall.
 34. A prostheticdevice comprising: a fixation member sized to fit within a cavity in afirst vertebral body; and a compressible member comprising: a basemember; an upper member that includes a spike; and one or morecompressible elements provided between the base member and the uppermember, wherein the one or more compressible elements are sized tosubstantially replace a nucleus pulposus of an intervertebral discadjacent the vertebral body.
 35. The prosthetic device according toclaim 34, wherein the base member of the compressible member isconfigured to engage the fixation member and the upper member of thecompressible member is configured to engage a second vertebral body. 36.A prosthetic device comprising: a fixation member sized to fit within acavity in a first vertebral body; and a compressible member comprising:a base member; an upper member comprising a ball-and-socket joint; andone or more compressible elements provided between the base member andthe upper member, wherein the compressible member is sized tosubstantially replace a nucleus pulposus of an intervertebral discadjacent the vertebral body.
 37. The prosthetic device according toclaim 36, wherein the upper member further comprises a plate supportingthe ball-and-socket joint, and wherein the ball-and-socket jointcomprises a spike.
 38. The prosthetic device according to claim 36,wherein: (i) the base member of the compressible member is configured toengage the fixation member, and (ii) the upper member of thecompressible member is configured to engage a second vertebral body. 39.The prosthetic device according to claim 36, wherein the ball-and-socketjoint is configured to be fixed or to remain mobile upon implantation.40. A prosthetic device comprising: a fixation member sized to fitwithin a cavity in a first vertebral body; and a compressible membercomprising: a base member; an upper member; and one or more compressibleelements provided between the base member and the upper member, wherein:(A) the compressible member is sized to substantially replace a nucleuspulposus of an intervertebral disc adjacent the vertebral body, and (B)the base member is adjustable in a radial direction.
 41. The prostheticdevice according to claim 40, wherein the base member is adjustable bymeans of a rotatable plate having a plurality of projections.
 42. Theprosthetic device according to claim 41, wherein when the rotatableplate rotates, the projections engage and radially extend a plurality ofextendable leaves.
 43. A method of spinal prosthetic implantation, themethod comprising the step of: creating a cavity in a first vertebralbody; cutting a first hole through either a lower or an upper endplateof the vertebral body and through the nucleus pulposus of a firstintervertebral disc adjacent thereto, thereby creating a first openingin the first intervertebral disc; cutting a second hole through theother of the lower and upper endplate of the vertebral body and throughthe nucleus pulposus of a second intervertebral disc adjacent thereto,thereby creating a second opening in the second intervertebral disc;implanting a first compressible member into one of the first and secondopenings; implanting a second compressible member into the other of thefirst or second openings; and implanting a fixation member into thecavity in the first vertebral body.
 44. The method according to claim43, further comprising the step of adjusting a length of the fixationmember.
 45. The method according to claim 43, further comprising thestep of creating a first seat in an endplate of a second vertebral bodybelow the first vertebral body.
 46. The method according to claim 45,further comprising the step of creating a second seat in an endplate ofa third vertebral body above the first vertebral body, wherein each ofthe first and second seats is configured to receive a respective one ofthe first and second compressible members.
 47. The method according toclaim 43, further comprising the step of positioning, in the fixationmember, at least one of bone allograft, bone cement, and a bioresorbablepolymer.
 48. The method according to claim 43, wherein the step ofimplanting a first compressible member into one of the first and secondopening comprises providing a plurality of compressible elements in thefirst compressible member, wherein the plurality of compressibleelements vary in a degree to which they are compressible.
 49. The methodaccording to claim 43, wherein: (i) the first compressible membercomprises a base member that is wider than the hole cut in the vertebralbody through which the first compressible member is implanted, and (ii)the step of implanting the first compressible member into one of thefirst and second openings comprises the steps of: (a) maneuvering thebase member of the first compressible member so that it passes throughthe hole in the one of the first and second openings; and (b) rotatingthe base member so that it substantially covers the hole.
 50. The methodaccording to claim 43, wherein: (i) the first compressible membercomprises a base member that is radially adjustable to be wider than thehole cut in the vertebral body through which the first compressiblemember is implanted, and (ii) the step of implanting the firstcompressible member into one of the first and second openings comprisesthe steps of: (a) maneuvering the base member of the first compressiblemember so that it passes through the hole in the one of the first andsecond openings; and (b) radially expanding the base member so that itsubstantially covers the hole.
 51. A method of spinal prostheticimplantation, the method comprising the steps of: creating a cavity in afirst vertebral body; cutting through an endplate of the vertebral bodyand through the nucleus pulposus of an adjacent intervertebral disc,thereby creating an opening in the intervertebral disc; cutting into thecortical bone of a second vertebral body on the other side of theintervertebral disc to create a seat; implanting a compressible memberinto the opening in the intervertebral disc such that a distal end ofthe compressible member sits within the seat in the second vertebralbody; and implanting a fixation member in the cavity in the firstvertebral body.
 52. The method according to claim 51, further comprisingthe step of adjusting a length of the fixation member.
 53. A method ofspinal prosthetic implantation, the method comprising the step of:creating a cavity in a vertebral body; cutting a hole through either alower or an upper endplate of the vertebral body and through the nucleuspulposus of an intervertebral disc adjacent thereto, thereby creating anopening in the intervertebral disc; implanting a compressible memberinto the opening in the intervertebral disc; and implanting a fixationmember into the cavity in the first vertebral body, wherein: (i) thecompressible member comprises a base member that is wider than the holecut in the vertebral body through which the first compressible member isimplanted, and (ii) the step of implanting the compressible member intothe opening comprises the steps of: (a) maneuvering the base member ofthe compressible member so that it passes through the hole and into theopening; and (b) rotating the base member so that it substantiallycovers the hole.
 54. The method according to claim 53, furthercomprising the step of creating a seat in an endplate of a secondvertebral body on the other side of the intervertebral disc in which theopening is formed.
 55. A method of spinal prosthetic implantation, themethod comprising the step of: creating a cavity in a vertebral body;cutting a hole through either a lower or an upper endplate of thevertebral body and through the nucleus pulposus of an intervertebraldisc adjacent thereto, thereby creating an opening in the intervertebraldisc; implanting a compressible member into the opening in theintervertebral disc; and implanting a fixation member into the cavity inthe first vertebral body, wherein: (i) the compressible member comprisesa base member that is radially adjustable to be wider than the hole cutin the vertebral body through which the compressible member isimplanted, and (ii) the step of implanting the compressible member intothe opening comprises the steps of: (a) maneuvering the base member ofthe compressible member so that it passes through the hole and into theopening; and (b) radially adjusting the base member so that itsubstantially covers the hole.
 56. The method according to claim 55,further comprising the step of creating a seat in an endplate of asecond vertebral body on the other side of the intervertebral disc inwhich the opening is formed.
 57. A drill guide for use in spinalsurgery, comprising a body having a first leg and a second leg, wherein:(A) the first leg is dimensioned to be fixed relative to anintervertebral prosthetic member mounted in a cavity of a firstvertebral body, (B) the second leg is dimensioned to extend from thefirst leg, adjacent the first vertebral body, to a free end, adjacent atleast one of an intervertebral disc and a second vertebral body, and (C)the second leg includes at least one drilling channel extending throughthe second leg and the free end of the second leg comprises a drillpositioning block through which the at least one drilling channelextends.